The present invention relates generally to the transmission of digital data. More particularly, the invention relates to a high-speed data transmission between ICs (integrated circuits).
Over past years, the operating speed of computer processors has increased dramatically, resulting in additional demands on speeds of transmission of digital data. This has lead to very high frequencies in data transmission lines. At high frequencies some effects occur which are not observed at low frequencies in connection with the behavior of passive circuit elements (wires, circuit boards and integrated-circuit packages that make up a digital product). That is, at high frequencies passive circuit elements directly affect electrical performance.
One high frequency effect of particular concern in the present invention is that of signal reflection, which occurs when the impedance of a transmission line does not match the impedance of a terminating load on the driving end or receiving end of the line. Ideally, a terminating load will sink a transmitted signal immediately upon the signal arriving at the load. However, the terminating load often does not match the transmission line impedance because of variations in output resistance, which can result from the production variations of elements and/or temperature. In this case the load will sink only a portion of the signal upon the signal""s initial arrival. The remaining portion of the signal will be reflected back onto the transmission line. If the driving circuit provides a terminating load that matches the impedance of the transmission line, the reflected signal portion will sink on reaching the driving end. Otherwise, the reflected signal portion will be partially reflected again and thus return to the receiving end. Thus, substantial reflected signal portions can go back and forth until they damp in the transmission line. Every time the reflected signal portion comes to the receiving end, it affects the main signal by producing an additional skew. This reduces the precision with which the signal can be measured, and therefore does not allow increasing the speed of data transmission.
It shall be noted that, within the above discussion, a uniform transmission line was considered, that is, a transmission line having impedance which can change in time, not lengthwise. However, a transmission line is generally not uniform. It can include unequal-impedance parts such as conductive traces on different boards or in different layers of the same board, connectors, and/or cables. An interface between these parts will also produce reflections, which will propagate both to the driving and receiving ends of the transmission line and contribute in affecting the signal.
Efforts were made to eliminate, or at least greatly reduce, signal reflections by matching, insofar as possible, the impedance of a terminating load and that of a transmission line.
Known are various impedance-matching means disclosed in U.S. Pat. Nos. 4,719,369; 5,134,311; 5,162,672; 5,811,197; 5,602,494; 6,191,663; 6,175,250; 6,157,215; 6,130,563; 6,127,862; 6,118,310; 6,087,853; 6,060,907; 5,955,894 and 5,578,939.
Generally, these patents disclose IC output drivers comprising a circuit which compensates for the variations in output resistance.
Specifically, a typical digitally controlled output driver disclosed in U.S. Pat. No. 6,118,310 has a pull-up and pull-down predriver circuits driven by an impedance control circuit. In operation, the driver impedance is pulled up or down as needed to match with the impedance of a transmission line. With the driver according to U.S. Pat. No. 6,118,310, a portion of the signal reflected on the receiving end back onto the transmission line will sink on its driving end and thus will not affect the received signal. However, the driver according to U.S. Pat. No. 6,118,310 will not help in sinking the above-mentioned reflections produced at the interfaces between the unequal-impedance parts. These reflections will go back and forth between the interface and the receiving end of the line, thus not reaching the driver end where they can be terminated.
In U.S. Pat. No. 5,578,939 a bi-directional transmission line driver/receiver for transmitting multi-valued logic signals is disclosed. The idea of the patent is to set interaction between the driver and the receiver by exchanging reference signals setting the magnitude of the signal to be transmitted and a bias of the terminator provided at the receiver end of the line. These reference signals are transferred via an additional transmission line referred to as xe2x80x9creference linexe2x80x9d. The additional line and interaction means make it a relatively complex system which will not, however, sink reflections produced at the interfaces between the unequal-impedance parts and directed backwards to the driving end of the transmission line. Reflected again at the driving end, these reflections will come to the receiving end and affect the signal.
The object of the invention is to provide a relatively simple transmission system and method which will effectively eliminate both the reflections produced on the ends of a transmission line and the reflections resulting from discontinuities within the transmission line, and will thus allow increasing the speed of data transmission.
In one aspect of the invention, claimed is a transmission system for transmission of digital data, the transmission system including a transmission line having a driver end connected to a driving circuit and a receiving end connected to a receiving circuit, each said end having an adjustable termination means connected thereto, wherein on the driver end of the transmission line said adjustable termination means is incorporated in the driving circuit, while on the receiver end of the transmission line said adjustable termination means is connected in parallel with the receiving circuit. Thus, both the reflections produced on the ends of a transmission line and the reflections resulting from discontinuities within a transmission line will be terminated.
The adjustable termination means can include a test line having substantially the same impedance as at least a portion of the transmission line. The test line can be constituted by the transmission line.
The driving circuit preferably comprises a buffering circuit connected to the driver end""s adjustable termination means which in turn includes an impedance adjusting circuit and an associated control circuit.
The test line can be incorporated in the control circuit. Preferably, connected to the test line, is a test circuit including a buffering circuit and impedance adjusting circuit of substantially the same structure as the buffering circuit and impedance adjusting circuit incorporated in the driving circuit.
The test circuit can be constituted by a driving circuit of the transmission line.
The control circuit can further include a pulse generator, a counter, a comparator, and a flip-flop clocked by the pulse generator with a delay, the output of the pulse generator and the counter can be connected to the buffering circuit and impedance adjusting circuit, respectively, the test line and a source of a reference voltage can be connected to the input of the comparator, the output of the comparator can be connected to the flip-flop, and output of the counter can also be connected to the impedance adjusting circuit of the driving circuit.
Preferably, the receiver end termination means is constituted by a termination circuit which has substantially the same structure as said driving circuit. This allows fabrication of the data transmission circuit in a modular structure to reduce its cost.
In another aspect of the invention, claimed is a method of data transmission through a transmission line, the method including the following steps:
feeding, via a test circuit, a test pulse to a test line having the same impedance as at least the portion of the transmission line;
comparing the test pulse with a reference signal of half the voltage of the driver swing;
adjusting the test circuit impedance to the value at which the test pulse substantially equals the voltage of the reference signal;
setting the impedance of the driver of the transmission line to the said value.
The test pulse can be fed to the transmission line used as the test line. Thus, no separate test line is needed. In this embodiment data transmission shall be interrupted to feed the test pulse to the transmission line.
An exemplary embodiment of the invention is further described, by means of example, in detail with reference to the attached drawings.